Tom-- Thank you!  Now that I've actually read the manual you referenced, it's obvious how to handle OUT parameters too:

SQL> ed
Wrote file afiedt.buf

  1  create procedure sp_test_dynamic_parms ( X IN NUMBER, y OUT NUMBER ) AS
  2           BEGIN
  3              y := X + 1;
  4*          END;
SQL> /

Procedure created.

SQL> DECLARE
  2    a number;
  3    b number := 1;
  4  BEGIN
  5    execute immediate
  6       'BEGIN sp_test_dynamic_parms ( :1, :2 ); END;'
  7                             USING IN b, OUT a;
  8    DBMS_OUTPUT.PUT_LINE('result = '||a);
  9  END;
 10  /
result = 2

PL/SQL procedure successfully completed.
 

Of course, I'd be remiss if I didn't follow "Professor Tom"'s advice to TKPROF compare dynamic and static calls. I found these in the dynamic call portion:

declare a number; begin
for i in 1..100000 loop
execute immediate
'BEGIN sp_test_dynamic_parms ( :1, :2 ); END;'
USING IN i, OUT a;
DBMS_APPLICATION_INFO.SET_ACTION('@'||i);
end loop;
END;

call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.01 0.05 0 0 0 0
Execute 1 23.84 35.68 0 2 0 1
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 23.85 35.74 0 2 0 1

BEGIN sp_test_dynamic_parms ( :1, :2 ); END;

call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 100000 11.25 16.26 0 0 0 0
Execute 100000 17.50 25.88 0 0 0 200000
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 200000 28.76 42.15 0 0 0 200000

>>>>>>> versus <<<<<<<<<

declare a number; begin
for i in 1..100000 loop
sp_test_dynamic_parms ( i, a );
DBMS_APPLICATION_INFO.SET_ACTION('@'||i);
end loop;
END;

call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.01 0.00 0 0 0 0
Execute 1 2.95 3.74 0 0 0 1
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 2.96 3.74 0 0 0 1

I assume the summation of rounding errors explains the "greater" elapsed time reported for the 100000 calls to
BEGIN sp_test_dynamic_parms ( :1, :2 ); END;
than for the single anonymous block that made the calls.